: The long-range goals of this research are to discover the role of the posterior parietal cortex in visuospatial operations, and to examine the effects of attention on neurons responsible for processing visuospatial information. New knowledge of these processes will lead to a better understanding of higher order sensory processing in the brain, and of the complex syndrome observed in subjects with parietal lesions that have difficulty responding to visual cues and performing motor operations in the extrapersonal space on the side opposite and lesion. The activity of individual neurons area PG of the parietal cortex of behaving monkeys is related to a variety of phenomena including attentive fixation, visual tracking or saccading, reaching movements to targets, and to purely visual aspects of stimuli. The first objective of this proposal is to test the hypothesis that the activity of parietal visual neurons (PVNs) encodes the relative separation of two simultaneously presented stimuli, independent of their absolute location in the visual field. This hypothesis will be tested in monkeys trained to detect the spatial relationships between visual stimuli. The rates of discharge of PVNs will be recorded and compared, when two stimuli that vary in spatial separation and absolute location within the visual field are presented to the animal. Experiments have demonstrated enhanced responses of PVNs to passive visual stimuli when attention is directed toward a central fixation target. The second objective is to determine the importance of behavioral relevance of visual stimuli on PVN responses. This will be tested by comparing PVN responses to identical visual stimuli presented under three conditions of attention: (1) alert but not attending, (2) attending to a fixation target but not to the stimulus, and (3) attending to the stimulus. Parietal neurons of the different classes described above are organized into columns, but little is known about the arrangement of these columns. The third objective of this research is to determine how columns of neurons in the posterior parietal cortex are arranged with respect to other columns of the same or different class. This will be determined by constructing relatively fine-grained maps of areas PG based on neuronal responses in monkeys trained to perform relevant visual, oculomotor, and reaching behaviors. These data will be examined to determine if there exists an organized representation of extrapersonal space in the parietal lobe.